Ion exchange (“IX”) and electrochemical methods and devices using ion exchange structures were initially developed more than 50 years ago, and have since that time been improved to the point that such systems are commonly employed to purify fluids for a variety of applications. Typically, IX and electrochemical membrane methods and devices such as electrodialysis (“ED”) and reversing type electrodialysis (“EDR”) purify fluid through ion exchange or electric field-mediated transfer of ions through membranes from diluting or permeate streams passing through “less concentrated” compartments to concentrating or brine streams passing through “more concentrated” compartments. Generally, anion transfer and cation transfer membranes are alternated in ED methods and devices, the membranes being placed between an anode (positive electrode) and a cathode (negative electrode) across which a DC electric field is applied transverse to the fluid flow directions. Anion transfer membranes allow passage only of low molecular weight negatively charged species (anions), and cation transfer membranes allow passage only of low molecular weight positively charged species (cations). Transfer of ions across membranes is mediated by the attraction of anions to the anode and cations to the cathode. The combination of an anode, a cathode, and alternating anion and cation transfer membranes therebetween is commonly referred to as an ED “stack” or pack.
FIG. 1 depicts a schematic view of an exemplary ED unit 10 having a cathode 12 and an anode 14 and cation transfer membranes 20 alternating with anion transfer membranes 22. Cation transfer membranes 20 and anion transfer membranes 22 form a plurality of alternating ED diluting compartments 24 and ED concentrating or brine compartments 26. A fluid, for example water, enters the ED unit 10 at electrode stream inlets 52 and exits ED unit 10 at electrode stream outlets 54 to form electrode streams 50. The electrode streams 50 that comes into contact with cathode 12 or with anode 14 do not mix with, and are not in fluid communication with, fluid in ED feed stream 30 or with fluid in ED brine stream 40 (see below).
Fluid to be purified flows into ED unit 10 in the form of ED feed stream 30 which enters the unit at ED feed stream inlet 32. ED feed stream inlet 32 is in fluid communication with ED feed stream inlet manifold 34, through which fluid to be purified is delivered to one or more ED diluting (less concentrated) compartments 24. The number of diluting compartments 24 in an ED unit can vary according to the application in which the ED unit is used. Determinations of the appropriate number of diluting compartments for a particular application can be accomplished empirically, on the basis of the desired capacity of the fluid purification system and the amount and identity of contaminants in the feed stream. As defined herein, diluting compartment 24 of ED unit 10 involves the sum of all diluting compartments contained within the unit. After traversing the diluting compartment 24, fluid from ED feed stream 30 enters ED product stream outlet manifold 36, exiting the ED unit as less concentrated product stream 30a at ED product stream outlet 38. Fluid is purified in the ED diluting compartments 24 by virtue of passage of ions out of the ED diluting compartments 24 into the more concentrated ED concentrating or brine compartments 26.
In parallel to the flow of ED feed stream 30, an ED concentrate or brine influent 40 flows into unit 10 at ED concentrate or brine stream inlet 42. ED concentrate or brine stream inlet 42 is in fluid communication with ED concentrate or brine stream inlet manifold 44, through which fluid that receives ions from the ED diluting compartments 24 is delivered to one or more ED concentrating or brine compartments 26. The number of concentrating or brine compartments in an ED unit may vary according to the application in which the ED unit is used, but will be equal to (or ±1) the number of diluting compartments in the unit. In accordance with the invention, concentrating or brine compartment 26 of ED unit 10 comprises the sum of all concentrating or brine compartments contained within the unit. After traversing the ED concentrating compartment 26, fluid from ED concentrate or brine influent 40 enters ED concentrate or brine stream outlet manifold 46, exiting the ED unit at ED concentrate or brine stream outlet 48. After exiting from ED concentrate or brine stream outlet 48, all or at least a portion of the brine stream is discarded as “blowdown”, and the remainder, if any, of the effluent brine stream is recycled into concentrate or brine influent 40, upstream of brine stream inlet 42.
Anion selective polymers for use in anion exchange resins or transfer membranes involved in the electrochemical devices and processes described above may be manufactured via a variety of techniques. For example, anion selective polymers may be prepared by co-polymerizing methacrylate esters containing amine groups of the tertiary type, with cross-linking methacrylate esters (see for example U.S. Pat. No. 4,231,855 by Hodgdon et al.). The resulting polymer with pendant tertiary amine groups may be quaternized with an alkyl halide, such as methyl chloride, so that the tertiary amine groups are converted to quaternary ammonium salts.
The above-described technique may require washing steps between process steps and requires chemical reactions on polymerized sheets. Further, exchange resins and transfer membranes formed of methacrylate esters may degrade rapidly in the presence of caustic solutions. In addition, exchange resins and transfer membranes formed by the above-identified technique may lack resiliency and further, the membranes may leak, because the post-polymerization quaternization reactions may weaken the resin.
Anion selective polymers for use in anion exchange resin particulates or transfer membranes employed in electrochemical devices and methods may also be prepared by solubilizing a cross-linking monomer such as methylene bisacrylamide (MBA), by pre-treatment with a caustic solution. The solubilized MBA may then be combined with an acrylic monomer, such as dimethylaminopropylmethacrylamide, in a water soluble solvent and polymerized. See U.S. Pat. Nos. 5,037,858 and 5,354,903 to MacDonald. As in the previously identified technique, the resulting polymer may have to be further reacted so that its pendant tertiary amine group is converted to a quaternary ammonium salt to form the anion selective polymer. Alternatively, these patents teach combining the solubilized MBA with an ionogenous acrylic monomer which has already undergone quaternization, such as methacrylamido-propyltrimethylammonium chloride, in a water soluble solvent and polymerizing the liquid mixture.
Accordingly, the above-identified technique requires a caustic solution pretreatment step for solubilizing the cross-linking monomer. Further, as in the previously described technique, post-polymerization quaternization may weaken the exchange resins and transfer membranes made from such polymers. The alternative process involving use of an ionogenous acrylic monomer requires a special solvent to prevent the quaternary ammonium salt from precipitating out of the liquid solution before the polymerization occurs.
Thus, there is a need to develop caustic stable anion exchange resins and transfer membranes with resilient surfaces and substantially leak-free transfer membranes for use in ion exchange and electrochemical methods and devices. Further, there is a need to develop simplified methods of forming such exchange resins and transfer membranes which avoid precipitation resulting from quaternization prior to polymerization. In addition, there is a need to develop simplified methods of forming such exchange resins and transfer membranes which avoid the necessity of washing steps associated with polymer formation prior to quaternization.